Posted: 24/09/2015 at 18:43
Looks like a fungal infection but that could be an after effect following attacck by some kind of sap-sucking pest, with aphids, whitefly or flea beetle being the most likely. If you haven't done it for a while, I would try dosing whichever section of ground you will be using next year with garden lime this winter. This will reduce the acidity and in doing so will reduce the likelihood of pests and diseases as most prefer acidic conditions. Brassicas also prefer slightly alkaline soil. I would also give the area a good sprinking of fish, blood and bone next spring before you plant anything out, just in case it is a nutrient deficiency of some kind.